1. Field of the Invention.
The present invention relates to a method for joining together multiphase objects, and more particularly, the present invention relates to methods for joining together ceramic shapes to form pore-free joints or junctions at least as strong as the materials being joined, and to join ceramic/metal composites (cermets).
2. Background of the Invention
Ceramics in general are difficult to form into complex shapes. At present, complex ceramic shapes are often prepared by forming the complex shape in the green state and then applying heat to consolidate the shape. This can be difficult and costly.
An alternative method is to form a ceramic blank and then machine the blank into the desired shape. This method also can be difficult, time consuming, expensive, and can introduce faults in the structure, which will reduce performance of the structure significantly or possibly render the shape unusable.
Another alternative is to join simpler shapes to form the desired complex shapes. This alternative is similar to brazing or welding of various metal components to form more complex metal components. Ceramic components have been joined using various glasses and metals as the joining material. However, the resulting joints have poor mechanical properties compared to the materials to be joined, and the application temperatures are limited.
Nanocrystalline materials have been used as joint-forming interlayer constituents between the two ceramic shapes to be joined. F. Gutierrez-Mora, A Dominguez-Rodriguez, J. L. Routbort, R. Chaim, and F. Guiberteau, “Joining of yttria-tetragonal stabilized zirconia polycrystals (Y-TZP) using nanocrystals,” Scripta Mater., 41, 455-460 (1999). However, nanocrystalline materials can be very expensive and can be difficult to work with. Nanocrystalline ceramics are difficult to consolidate into a dense body without having the individual grains grow substantially. Growth of the grains eliminates the nanocrystalline nature of the body and renders another common ceramic.
Nanocrystalline materials have other drawbacks. For example, such powders tend to agglomerate badly, hence making it difficult to produce dense bodies or to apply materials uniformly to a surface. Also, nanocrystalline powders are often highly hygroscopic. Adsorption of moisture can make it difficult or impossible to process nanocrystalline ceramic powders into strong, pore-free bodies.
A process called superplastic deformation has been used to join Y-TZP of the same composition as that mentioned in the F. Gutierrez-Mora reference, supra.; J Ye. et al., Scripta Metall. Mater., 33, pp 441-445 (1995); and A. Dominguez-Rodriquez, et al, J. Mater. Res., 39, p1631-1636 (1998). Superplasticity in deformation of materials usually occurs by a process known as grain-boundary sliding. At elevated temperature, under application of a stress, individual grains of the solid slide and rotate past each other so that permanent deformation can take place. Generally for superplasticity to occur, individual grains must remain virtually stable. They cannot grow or change shape significantly, nor should they react with other species present. If grain growth occurs, superplasticity is prevented. As grains grow during deformation, small pores are created. These pores grow and eventually join to form cracks which reduce the ceramic strength.
Many previous efforts using nanocrystalline materials to join objects have resulted in a joint containing inferior, stable residual porosity. R. Chaim et al., J. Mater. Res., 15, pp 1724-1728 (2000). Thus, in these and other similar instances, an expensive, difficult-to-prepare interlayer was required. Either that, or settle for an inferior joint.
Aside from joining ceramic forms, it is also desirous to join shapes comprised of cermets. Cermets are ceramic/metal composites in which ceramic particles are the majority phase by volume. Most cermets contain between 5 and 15 volume percent metal to bind hard ceramic particles such as WC and TiC. Cermets are employed in applications such as cutting tools in which wear resistance is required.
With respect to joining simple cermets to form more complex structures, such as, for example, serrated cutting edges, no joining technology has been found to be widely successful. Conventional welding is ineffective because, at welding temperatures, the ceramic is solid but the metallic phase is liquid. Leaching of the metal and destruction of the cermet occurs during welding.
Conventional brazing or soldering forms joints with insufficient strength for many applications. Also, the resulting joints have poor resistance to heat. Because cutting and grinding operations often produce substantial heating of the cermet tooling, and the stresses on the tooling are high, brazed or soldered joints will fail in most applications.
Complex cermet tooling is typically fabricated to shape. The required procedures make use of intricate and relatively expensive dies or diamond-tooling machining. Furthermore, because of geometric constraints associated with part removal from a die, the shapes that can be formed are limited.
U.S. Pat. No. 6,168,071 awarded to Johns on Jan. 2, 2001 discloses a method for joining materials together by a diffusion process using silver/germanium alloys. No external pressure is applied.
U.S. Pat. No. 5,855,313 awarded to McAfee et al. on Jan. 5, 1999 discloses a method for a two-step brazing process for joining materials with different coefficients of thermal expansion.
U.S. Pat. No. 5,599,419 awarded to Hunter et al. on Feb. 4, 1997 discloses a method for joining plastic materials via a heated blade which simultaneously heats the two surfaces to be joined. The heated blade is removed and the two surfaces are welded together.
U.S. Pat. No. 4,927,475 awarded to Steinleitner et al. on May 22, 1990 discloses a method for joining surfaces of different materials by applying a glass coating to the two surfaces to be joined. The surfaces are subsequently joined while heated and under external pressure.
U.S. Pat. No. 4,414,166 awarded to Charlson et al. on Nov. 8, 1983 discloses a method for laser joining of thermoplastic and thermosetting materials by laser radiant energy which causes the thermoplastic material to flow onto the thermosetting material. No external pressure is applied.
U.S. Pat. No. 4,247,345 awarded to Kadija et al. on Jan. 27, 1981 discloses a method for joining sections of synthetic materials by placing a thermoplastic sealing composition in a gap between the materials to be joined and then binding the materials together with subsequent heating of the sealing composition. No external pressure is applied.
A need exists in the art for a ceramic joint forming process that results in joints as strong, or actually stronger than the materials joined. The process should be simple in that no elaborate surface preparation or application techniques are required. The process also should utilize common ceramic materials and readily available equipment to minimize costs. Joining temperatures should be as low as possible to minimize degradation of the ceramics being joined and to minimize the cost and complexity of the tooling needed to form the joints.
There is also a need for a technology for producing robust complex cermet forms from simpler ones. The technology should be simple, inexpensive, require few steps to complete, and require minimal surface preparation. Cermets are difficult to polish (inasmuch as they themselves are used to polish other materials), and so polishing of the cermets resulting from the technology should not be required.